Now, when I got white-listed, I can post it here:I have been trying the multisig BIP 16 using the latest 0.6.3 client on Ubuntu 64 bit.I was trying to create 2 from 3 sig address, but it behaves like 3 out of 3 sig address.

I have sent 0.1 BTC to the 3G... address directly from instawallet: 3dd40692d5594ee1302c3a6fce4c9086af3324dfadabf97150c8c5ea7ff78f1aas well as the following:$ bitcoind sendtoaddress 3G2v3VJZ46fjgLMqEF3CzLq3k9FPMgauh4 0.056c2f5a608fb3ef3e64e728ad430f60bbd6857de662b152dc2ef55eb25fdf2e93

The reference client only lets you spend coins sent to a multisig addresses if you have all involved keys. That doesn't mean that the protocol requires this.

The current functionality is really just proof of concept, as the question of who "owns" coins in a multisig setting is not very well defined. In practice, you want to use it in a situation where not all required keys are available to a single system at all, and a different mechanism is needed to create, gather and combine the required signatures.

The 0.7.0 release will come with a 'raw transaction' API that allows signing unsigned or partially signed transactions, passing them around out of band, and when fully signed, broadcasted to the network.

Pieter is exactly right. The current code is extremely conservative with multisig addresses, only counting them as yours if the wallet contains all the private keys.

Loosening that to considering them yours if you have enough keys to spend might happen, although I am worried that might cause vulnerabilities for applications that make the implicit assumption that if they have the key necessary to spend that means nobody else can possibly spend those coins after 6 confirmations. If it is a 1of2 multisig that wouldn't be true.

How often do you get the chance to work on a potentially world-changing project?